Worldwide concerns on environmental pollution, energy depletion, and climate change are compelling environmental engineers to expand their responsibilities from pollution clean-up to sustainable development of energy and environmental systems. One emerging direction is to transform wastewater infrastructure from simple treatment processes to integrated energy and valuable product recovery systems. Current wastewater treatment processes and membrane based desalination technologies are energy intensive due to the power demand for aeration, sludge treatment, and membrane operation. For example, it is estimated that every year, the U.S. uses approximately 57 Terawatt hours of electricity for wastewater treatment, accounting for 1.5% of the national total electricity production (equivalent to 5.4 million households' annual electricity use). A sustainable approach to wastewater treatment considers recovering the energy content of organic matters while simultaneously achieving treatment objectives because energy content embedded in wastewater is estimated to be about 2-4 times the energy used for water infrastructure in the U.S. This means it may be possible to make wastewater treatment self-sufficient.
Furthermore, improving water supply and quality in many places around the world would aid in mitigating many problems facing both developed and developing countries. The United Nations estimates that due to a global increase in population of 80 million people per year, an additional 64 billion cubic meters per year of freshwater is required. Lack of water could lead to the displacement of 24-700 million people, greater national insecurity, and world conflict. Inadequate water sanitation and supply has been linked to many diseases such as malaria, cholera and typhoid. The World Health Organization estimates that, with improvements to water supply, sanitation and hygiene 4%-75% of the global diarrhea disease burden could be prevented. It is apparent that increasing freshwater production would drastically improve humanity. The problem with increasing water supply is that energy is required for the production of all water, and water is required for the production of all energy. This phenomenon, known as the water energy nexus, thus far has prevented a sustainable method of producing energy or water. One clear indicator of the water energy nexus is that in the U.S., water used for cooling power plants equals the amount of water used for agriculture.
Currently the two main methods by which saltwater can be desalinated is with electrodialysis (ED) or reverse osmosis (RO). However, these technologies are not sustainable because of the substantial amount of external energy required. In 2008 a significant advance was made by the development of a microbial desalination fuel cells (MDC) which can desalinate water without any external energy. MDC technology uses microorganisms to oxidize a substrate, potentially municipal wastewater, to generate the energy required for desalination. The main problem with the MDC technology is that the ions from desalination become concentrated in the anode and cathode chambers. This concentration of ions in the anode and cathode chambers prevents MDC from being a sustainable method for desalination. If wastewater was used as the substrate, the increase in total dissolved solids (TDS) can prevent the treated wastewater from being reused.
With respect to wastewater, direct energy production from waste materials via bioelectrochemical systems (BESs) offers economic and environmental benefits because the energy produced offsets the energy consumption associated with treatment and reuse processes. BESs may use microorganisms to catalyze the oxidization of organic and inorganic electron donors in the anode chamber and deliver electrons to the anode. The electrons may be captured directly for electricity generation, in devices such as microbial fuel cells (MFCs). In other examples, the electrons may be supplemented by external power input for producing hydrogen, methane, or value-added chemicals in devices such as microbial electrolysis cells (MECs). The electrons may also be used in the cathode chamber to remediate contaminants such as uranium, chlorinated solvents, and perchlorate. The potential across the electrodes may, in other examples, also drive desalination through MDCs.
Compared to traditional environmental technologies, which generally provide one approach for pollutant control, bioelectrochemical systems offer both oxidation and reduction approaches for waste treatment, contaminant remediation, energy and water recovery. On the anode side, BESs can theoretically oxidize any biodegradable substrate and extract electrons to the anode. In addition to simple sugars and derivatives, many complex waste materials have been utilized such as wastewater effluents, biomass, landfill leachate, and petroleum hydrocarbons. On the cathode side, any electron acceptor type of contaminants can potentially be reduced using the electrons supplied from the cathode. Such contaminants include chlorinated solvents, perchlorate, chromium, uranium, etc.
An advantage of using BESs in wastewater treatment is its potential to convert traditional energy intensive treatment processes into energy gaining processes while still achieving treatment objectives. However, despite the great potentials BES offers in environmental engineering, the energy output highly depends on the degradability of the substrate, the reactor architecture, and the active microbial community. Though the power density from lab scale, acetate based reactors has increased from less than 1 mW/m2 to 6.9 W/m2 in the past decade, the power output from real wastewater is much lower compared to simple substrates due to the low biodegradability, conductivity, and buffer capacity in wastewater. For example, by using the same configuration of lab scale reactors, the maximum power density achieved from acetate (1.69 W/m2 or 42 W/m3) was more than 8 times higher than the power output from brewery wastewater (0.21 W/m2, or 5.1 W/m3) according to one test.
The restraints of wastewater in power production from BESs become more apparent in larger scale systems. Though the first 2 m pilot reactor has been operating since 2007 in Australia using brewery wastewater, the performance is reported to be unsatisfactory. One main reason identified is the low conductivity and alkalinity of the wastewater. The loss of electrons in the anode chamber results in the accumulation of protons, which will reduce the pH in anode chamber and inhibit microbial activity. Therefore, lab scale studies generally use high strength phosphate or carbon buffer solution (50-200 mM) to maintain pH neutrality. The buffer solution also provides additional conductivity to facilitate ion transfers to reduce system resistance. However, compared with buffer enhanced anolyte in lab studies, which keeps a neutral pH and high conductivity (˜20 mS/cm), real wastewater has a very low conductivity (1-2 mS/cm) and buffer capacity, leading to significant pH reduction and internal resistance increase that results in reduced power output from BES reactors. Because the continuous addition of buffer solution is costly and unsustainable, the nature of wastewater is one main challenge to be addressed before BES can be utilized on a large scale. Another approach to minimize the internal resistance is to reduce the distance between the electrodes. Porous separators such as J-cloth, glass fiber, and ion exchange membranes can reduce electrode spacing, provide electrode insulation, and decrease oxygen intrusion to improve electron recovery. Such separators are generally sandwiched between the anode and the cathode, but the reactor geometry becomes a challenge due to the risk of short circuit and deforming, especially when high surface brush anode was used. Tubular configuration with brush anode surrounded by a layer of cloth cathode is currently considered relatively feasible for larger scale reactors, but this configuration has been associated with a significant water leaking problems because the membrane/cathode assembly cannot hold the high static water pressure at larger scale. In addition, the low cathode surface area of the tubular design limited the power output.